Diabetes is a major cause of morbidity and mortality in the United States, with approximately 40 percent of all diabetic patients developing nephropathy which requires either kidney dialysis or transplantation. Specifically, diabetes mellitus is the leading cause of end stage renal disease and therefore, any newly diagnosed patient with diabetes mellitus would be considered at risk for the development of diabetic nephropathy. Several years prior to the development of renal insufficiency, diabetic patients also exhibit renal disease manifested by renal hypertrophy and hyperfiltration. Currently the only treatment suggested to prevent or reverse the renal hypertrophy of diabetic nephropathy is rigorous insulin therapy, although in practical application, insulin therapy has yielded disappointing results.
Under conditions of hyperglycemia, several normally minor pathways of glucose metabolism become more prominent. In the kidney these include the formation of sorbitol, the non-enzymatic glycosylation of proteins, and the de novo formation of diradyl glycerides. Beyer-Mears, A. L. et al., Diabetes 33:604-607 (1984); Brownlee, M., Annual Rev. Med. 42:159-166 1991); Craven, P. A. et al., Diabetes 39:667-674 (1990). These pathways are the basis for recent hypotheses on the mechanisms of diabetic complications, including nephropathy. In addition, increases in uridinediphospho-hexose (UPD-hexose) levels have been reported in the kidney and other organs in association with experimental diabetes. Cortes, P. et al., Kidney Int. 21:676-682 (1982); Spiro, M. J. Diabetologia 26:70-75 (1984). In particular, levels of uridinediphospho-glucose (UDP-Glc) were demonstrated to be consistently increased in early diabetes. This change has been postulated to increase the expression of glycoproteins and glycosaminoglycans in the basement membranes of diabetic tissues. Glycosphingolipids (GSLs), however, also require nucleotide-hexoses for their synthesis.
Glycosphingolipids are ubiquitous and complex molecules consisting of a lipid and a carbohydrate moiety. Glycosphingolipids are at the basis of several inherited metabolic disorders, and are also functionally important in organ development, growth, hormonal signalling and infection. Studies on the functional role of sphingolipids in the kidney and related tissue have focused mainly on their role in modulating growth through effects on phospholipase C and protein kinase C (PKC) activities. In particular, glucosylceramide (GlcCer) and related glycosphingolipids have been implicated in both the growth of cells and in the regulation of hormonal signaling.
In one study using Madin-Darby canine kidney (MDCK) cells, endogneous glycosphingolipid content was decreased by the glucosylceramide synthase inhibitor 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) and increased by the inhibition of .beta.-glucosidase with conduritol B epoxide. See Shayman, J. A. et al., J. Biol. Chem. 266(34):22968-22974 (1991). Decreased glycolipid formation was associated with a pronounced impairment of cell growth, whereas increased GlcCer formation induced cell proliferation. Decreasing glycolipid formation was also associated with a pronounced increase in hormone-stimulated inositol triphosphate (InsP3) formation, wherein the increased GlcCer formation had the opposite effect. Shayman, J. A. et al., J. Biol. Chem. 265:12135-12138 (1990); Mahdiyoun, S. et al., Arch. Biochem. Biophys. 292:506-511 (1992). These results implicated cellular GlcCer, or a metabolically close product of GlcCer metabolism, as a regulator of MDCK cell growth.
While glycosphingolipids have been generally studied as elements in cellular proliferation and regulation of hormonal signaling, and also implicated in tumor growth (see U.S. Pat. No. 5,041,441 to Radin et al.), they have not been previously linked to diabetic nephropathy. In fact, the potential pathogenic role of glycosphingolipids in the treatment of diabetic nephropathy has not heretofore been explored.
It would be desirable to develop an effective method for preventing diabetic nephropathy. It would also be desirable to develop an effective method for treating diabetic renal impairment. It would further be desirable to develop an effective method of reversing diabetic renal impairment.